1. Field of the Invention
The present invention generally relates to the field of control circuits used to power fluorescent lamps and in particular to those circuits which permit the illumination of a fluorescent lamp to be dimmed in response to a reduction in input power.
2. Prior Art
In order for a fluorescent lamp to illuminate, the voltage across the filaments thereof must be greater than the minimum voltage specified for the lamp. Under typical conditions, it is necessary for a lamp supply circuit to include a current limiting element. The fluorescent lamp itself acts as a voltage limiting component. The voltage across the filaments of a fluorescent lamp is independent of the power supply voltage and is determined by the power of the fluorescent lamp itself. Therefore, to illuminate a fluorescent lamp with conventional alternating current power (i.e., 115 volts/60 Hz), a ballast or current limiting component is utilized.
To turn on or illuminate a fluorescent lamp, it is necessary to utilize a triggering or starting component. This is generally referred to as a starter circuit which initially heats the filaments of the fluorescent lamp. The lamp is illuminated when the voltage across the filaments exceeds the minimum necessary for a particular fluorescent lamp. The prior art exhibits several basic circuits for powering fluorescent lamps. One of the conventional designs for a fluorescent lamp power supply circuit is shown in FIG. 1.
A fluorescent lamp 10 is a gas discharge tube, the inner surface of which is coated with a light-emitting substance, typically fluorescent or phosphorescent metallic salts (e.g., calcium tungstate, zinc sulphide or zinc silicate). The tube is filled with mercury vapor at extremely low pressure. FIG. 1 illustrates an exemplary fluorescent lamp 10. Excitation filaments F1 and F2 are placed at alternative ends of lamp 10 and are powered by an input alternating current voltage Vac. As can be seen in FIG. 1, input voltage Vac is applied at terminals 11 and 12. In the circuit shown in FIG. 1, the current limiting component is formed of a high value iron inductance L which is connected between terminal 11 and supply terminal 13 of filament F1. The second supply terminal 14 of filament F1 is connected to terminal 15 of filament F2 by way of a starter circuit 16 which is typically a thermal switch 19. Terminal 17 of filament F2 is connected to input terminal 12. A capacitor C interconnects power supply terminals 11 and 12.
A starter circuit 16 in the form of a thermal switch is used to heat up filaments F1 and F2 of lamp 10 by connecting together or otherwise short circuiting terminals 14 and 15 until the filaments F1 and F2 are no longer cold. The starter circuit 16 opens as soon as filaments F1 and F2 have reached a predetermined temperature. This will result in an over voltage which triggers or otherwise turns the fluorescent lamp 10 on by means of power stored as a result of inductance L.
Under normal operations, the function of inductance L is to limit the current in lamp 10 in order that it does not exceed the value for which it is designed. The function of capacitor C is to compensate for the dephasing associated with the inductive assembly in order to improve the power factor and to make lamp 10 acceptable for a connection to a network.
The disadvantages of the prior art circuit illustrated in FIG. 1 are inherent in its design. The design of the prior art illustrated in FIG. 1 is a conventional power system. The system uses a high inductor (e.g., 1 Henry) which will result in a structure which is cumbersome, bulky and heavy. Furthermore, the inductive nature of the assembly requires a capacitor C of high value (e.g., 10 xcexcF) which necessitates use of a heavy electrolytic capacitor. The primary disadvantage of the prior art circuit shown in FIG. 1 is that it cannot be used to dim the light emitted from a fluorescent lamp.
Another control circuit disclosed by the prior art is illustrated in FIG. 2. FIG. 2 constitutes a conventional electronic circuit which employs active components to limit the current drawn by the fluorescent lamp. The circuit illustrated in FIG. 2 employs a diode bridge D having a pair of input terminals 20 and 21 which are connected to terminals 22 and 23, respectively, of input alternating current voltage Vac. By means of a high value electrolytic capacitor C, the output terminal 22 of bridge D provides a direct current power source to a switched-mode converter 23 which is used to supply fluorescent lamp 24. Switched-mode converter 23 is a conventional circuit generally formed by a control circuit 25 which is associated with two MOS power transistors M1 and M2 which are connected in series between terminal 22 of bridge D and the ground, capacitor C being connected in parallel thereto. The terminal 30 of the switched-mode converter 23 is connected to a first terminal of a high frequency inductance L which is then connected in series with input terminal 19 of filament F1 of lamp 24. A capacitor C2 of low value interconnects terminals 26 and 27 of filaments F1 and F2 and enhances the ability to trigger or otherwise start fluorescent lamp 10.
Terminal 28 of filament F2 is connected to ground through a capacitor C3. Another capacitor C4 connects terminal 28 of filament F2 to input terminal 29 of switched-mode capacitor C5. Capacitor C4 and C5 are used to filter the direct current component in fluorescent lamp 24. Terminal 29 receives the direct current voltage provided by capacitor C1. Transistor M1 is connected between terminals 29 and 30 and transistor M2 is connected between terminal 30 and ground. Transistors M1 and M2 are controlled by circuit 14 which also includes a feedback input connected to terminal 30 and which is supplied from terminal 29 through resistor R. A capacitor C5 interconnects terminals 29 and 30 and contributes to the generation of an auxiliary power supply necessary for the control of transistor M1.
The disadvantage of the prior art circuit illustrated in FIG. 2 is that, as in the circuit shown in FIG. 1, it requires electrolytic capacitors of high value (e.g., more than 10 xcexcF) to filter the rectified voltage output at terminal 22. The use of electrolytic capacitors will result in the reduction of the life of the circuit. Another disadvantage of the circuit shown in FIG. 2 is that harmonics from the supplied current will affect the power factor in the absence of a correction circuit. As with the prior art circuit illustrated in FIG. 1, that shown in FIG. 2 cannot be employed for dimming the illumination of a fluorescent lamp.
The present invention substantially resolves the inadequacies inherent in the devices disclosed by the prior art. Principally, the present invention control circuit allows the illumination from a fluorescent lamp to be responsive to the input power and, most importantly, to permit illumination to commence at an input voltage which is less than that specified for the selected fluorescent lamp. The objectives of the present invention are achieved through the use of a half-wave voltage doubler circuit. The alternating current signal is processed to alter the sinusoidal form of the input alternating current power and apply voltage to the fluorescent lamp filaments only once per cycle. The effect of the present invention is to double the voltage that drives the fluorescent lamp. This will cause the fluorescent lamp to illuminate at a higher level and will permit the fluorescent lamp to be dimmed when the alternating current voltage input to the present invention is reduced even to that which is less than the minimum generally required for operation of the fluorescent lamp. When the input alternating current voltage is reduced, the light emitted from the fluorescent lamp will be reduced or dimmed accordingly.
The present invention comprises a control circuit or ballast for a fluorescent lamp which will permit the user to dim the illumination of the lamp. Rectified alternating current power is applied to a self-oscillating multi-vibrator circuit. The multi-vibrator circuit creates an oscillating, high frequency square wave which excites the resonant circuit. A sinusoidal voltage is magnified by the power factor at resonance. The voltage will increase until it reaches a sufficient amplitude to strike or otherwise start the operation of the fluorescent lamp.
The circuit driving the fluorescent lamp creates a half-wave voltage signal which is substantially double that of the input alternating current signal. By doubling the voltage of the input signal, the fluorescent lamp will illuminate at a brighter level and it will compensate for the illumination lost during the non-active portion of the signal. In addition, by doubling the voltage driving the lamp, the present invention circuit will cause the fluorescent lamp to commence operation at an input voltage which is lower than that specified for the fluorescent lamp.
It is an object of the present invention to provide an improved control circuit for dimming a fluorescent lamp.
It is another object of the present invention to provide a dimmable ballast for a fluorescent lamp which is compact.
It is still another object of the present invention to provide a dimmable ballast for a fluorescent lamp which incorporates a voltage-doubler circuit which permits operation of a fluorescent lamp at input voltages which are lower than that specified for the fluorescent lamp.
It is still yet another object of the present invention to provide a dimmable ballast for a fluorescent lamp which is simple and inexpensive to fabricate.